Equine Skin Allergies

equine allergies, horse hives, allergy testing horses, allergy shots horses, pastern dermatitis horses, chronic progressive lymphedema horses, ringworm horses, summer sores habronema in horses

A Scratch for Every Itch

By Amy Young, UC Davis Center for Equine Health

In this Article

Equine Skin Allergies - an Overview

Horses scratch for many reasons. They scratch themselves on fences, rub up against posts (and sometimes people), roll on the ground, and groom each other. A natural behaviour usually linked to social bonding, comfort, and relaxation, it can be heightened seasonally by shedding, sweating, or the presence of insects. However, when scratching becomes frequent enough to result in hair loss, broken skin, scabs, or if it disrupts eating or sleeping, it is time to talk to a veterinarian to determine if a skin allergy is to blame so appropriate treatments can be pursued.

Allergies are the result of immune reactions to proteins that the body identifies as foreign (allergens). They are often caused by repeated exposures to pollens, weeds, grasses, molds, insect bites, medications, and feeds (although food allergies in herbivores are rare). Equine skin allergies typically cause abnormal itching (pruritus) and hives (urticaria).

Breaking Out In Hives

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A horse with hives due to seasonal environmental allergens. Photo: Pam MacKenzie

Among domestic animals, horses are the most likely species to be affected by hives. These raised, round patches on the skin are not a disease themselves. Rather, they are skin lesions or reaction patterns that result from certain diseases and conditions.

Related: Sweet Itch in Horses...Flaxseed Can Help

skin allergy test horses, hives horse, horse bumps on skin, horse has allergies, skin allergy horses

Equine intradermal allergy test. Photo courtesy of UC Davis

Frequent causes for hives in horses include:

  • Atopic dermatitis (environmental allergy: pollens, molds, etc.)
  • Drugs and medications
  • Insect/mite bites or stings
  • Chemical contact (plants, dyes, detergents, soaps, insecticides, etc.)

Less common causes are:

  • Dermatophytes (ringworm)
  • Pemphigus foliaceus (auto-immune skin disease)
  • Stress
  • Vasculitis (including purpura hemorrhagica)
  • Pressure or trauma to the skin
  • Cold
  • Exercise
  • Feed

Hives typically appear suddenly and in large numbers. The most distinguishing characteristic is that they “pit” if you put pressure on them. It is important to try to determine the underlying cause of hives by evaluating the horse’s medical history in order to provide proper treatment. A seasonal itch, for example, would be most consistent with atopic dermatitis to pollens, whereas an itch that persists year-round is more likely to be a reaction to molds, barn dust, or feed. Episodes of itching that occur after topical treatments of shampoos, dips, etc. would be consistent with a contact allergy.

Related: When Horses Get Allergies

Allergy Testing and Allergy Shots

Intradermal tests (IDT) and serum (blood) allergy tests are often used to identify allergens responsible for hypersensitivity reactions. Similar to allergy tests in humans, IDT involves injecting extracts from pollens, molds, and other environmental substances into the skin, often in a grid pattern, and measuring injection site responses anywhere from 30 minutes to a few hours later. Serum allergy tests are not always in agreement with IDT. Studies have reported variable repeatability for either method across time and individual animals, which could be due to a variety of factors. Horses with atopic dermatitis and recurrent hives generally have a higher incidence of positive reactions than healthy horses.

Allergens identified as causing hypersensitivity reactions and interpreted in light of the horse’s known disease history, clinical signs, and elimination of other causes may then be used for hyposensitization therapy. Also known as allergen specific immunotherapy (ASIT), or “allergy shots,” hyposensitization therapy involves administering small doses of an allergen on a gradually increasing basis to stimulate an immunological response. A study that evaluated treatment of cases of atopic dermatitis at the UC Davis veterinary hospital over 17 years noted an approximate 70 percent success rate in hyposensitization in horses; a comparable study at the University of Pennsylvania reported similar results. Interestingly, whether the hyposensitization is based on IDT or serum tests, the success rate is approximately the same. There is now the option of using oral hyposensitization instead of injections.

The ultimate success of this type of therapy depends on adherence to instructions set forth by the veterinarian. This includes anticipating that injections will need to be administered (usually by the owner or the horse’s caretaker) for at least a year, even if decreased clinical signs are observed within a shorter period. Approximately 85 percent of horses that respond to the therapy need continued hyposensitization for life, although often at decreased frequency.

Scratches (Pastern Dermatitis)

A Problem No Matter What You Call It

Pastern dermatitis, also known as scratches, greasy heel, mud fever, mud rash, cracked heels, and dew poisoning, has been well documented in horses since the early 1800s. It is commonly characterized by inflammation of the skin on the back of the pastern. Regardless of which name is used, most horse handlers will likely have to deal with it at some point. It is important to note that what can start as a minor skin annoyance can become a painful chronic condition if not treated properly.

Related: How to Treat and Prevent Mud Fever

Horses with pastern dermatitis usually exhibit multiple lesions in the pastern area. Initially, patchy red skin (erythema), oozing, crusting, erosions, and ulcerations develop, followed by swelling (edema) of the affected limbs. The skin can be itchy and sensitive. Lameness is observed in some cases and can become severe.

Symptoms Pastern Dermatitis horses, scratch horses

Clinical signs of scratches include inflammation and lesions in the pastern area (above: Pam MacKenzie; below: Shutterstock/Anjajuli

Symptoms Pastern Dermatitis horses, scratch horses

Although pastern dermatitis can occur in any horse breed, it is most common in draft breeds and others with heavy feathering on the lower legs that can trap moisture. Horses with white legs are also more susceptible as unpigmented skin is sensitive to sun damage.

It's Complicated

Pastern dermatitis is a clinical descriptive term for a disease complex that can be triggered by different causes. These can include environmental factors, allergies, drug reactions, fungi, bacteria, parasites, and sunlight-induced vascular changes of white legs. Extended exposure to moisture, in the form of wet bedding, muddy pastures, etc., seems to be a common cause or perpetuating factor. Bacterial infections typically develop due to a damaged skin barrier. Underlying genetic components are considered for draft horses with chronic progressive lymphedema. 

As there are many potential causes for pastern dermatitis, there are also a number of possible treatments. Diagnosis is based on biopsy for histopathology and bacterial and fungal cultures, but antibiotic therapy, accompanied by clipping and mildly cleansing the affected area (with the addition of topical corticosteroid application after one week if no improvement), is acceptable initially. Trimethoprim-sulfa antibiotics, corticosteroids, and/or resolution or control of the underlying disease process are the most common treatment modalities. In some cases, avoiding sun exposure may be helpful. Early, accurate diagnosis and appropriate treatment result in the most successful outcomes and prevention of severe chronic changes.

Related: Flies Affecting Horses: Buzzing and Biting

An Ounce of Prevention

Prevention of pastern dermatitis generally relies on avoiding exposure of the area to recurrent moisture. This includes limiting turnout of horses in wet grass or muddy pastures, drying the pastern areas well after bathing, keeping hair clipped on the lower legs, and avoiding long-term use of boots and wraps that can trap moisture. Proper stable hygiene, including providing clean, dry bedding, is essential for preventing pastern dermatitis and other health and welfare issues. 

Chronic Progressive Lymphedema

Chronic progressive lymphedema (CPL) is a debilitating condition caused by a buildup of lymph fluid (lymphedema) in the lower legs that results in progressive swelling. Over time, the swelling becomes firm and is associated with skin folds, nodules, and ulcerations. It has been described in several draft breeds, as well as Friesians and Gypsy Vanners. Since affected horses typically develop secondary recurrent bacterial and/or parasitic pastern dermatitis, CPL is often erroneously referred to as “chronic pastern dermatitis.” However, CPL is the underlying condition, which worsens with each recurrence of pastern dermatitis. 

Chronic Progressive Lymphedema in horses, infections horses

Chronic progressive lymphedema in a Belgian draft horse (left) and a Friesian (right). Both show marked nodular changes and evidence of secondary infection. Photos courtesy of UC Davis.

There is currently no successful permanent treatment for CPL. Careful management, supportive therapy, and avoidance of secondary infections can improve the quality of life for affected horses. This involves antibiotics to treat secondary bacterial infections; antiparasitic treatments to avoid reinfestation with Chorioptes mites; keeping the feathers clipped short; daily exercise; routine foot, ergot and chestnut trimming; daily hoof cleaning; and combined decongestive therapy (CDT). The latter is very helpful and includes manual lymph drainage massage and compression bandaging; both must be applied correctly and should only be performed by professionals. Horses should be kept in dry environments and pesticide applications may be required in barns to minimize mite infestations. These treatments are labour-intensive and must be maintained for the life of the horse to minimize discomfort, slow the progress of the disease, avoid recurrent infections, and ensure quality of life.

Related: What Are You Vaccinating Your Horse Against?

Researchers hypothesize that CPL is multifactorial. The wide distribution within each of the affected breeds suggests an underlying genetic component. “Several attempts to identify genetic factors of CPL have not been successful to date,” says Dr. Verena Affolter, a dermatopathologist at the UC Davis veterinary hospital who studies CPL together with Dr. Danika Bannasch, a geneticist at UC Davis. “As such, there is currently no clear path to prevent the disease in susceptible breeds.” Due to the late age of onset, many horses are bred prior to diagnosis, thereby passing any genetic predisposition to future generations.

Ringworm in Horses (and Its Look-alikes)

Looks Can Be Deceiving

On the surface, the term “ringworm” sounds straightforward. Although it can appear as a ring-like mark on the skin, it has nothing to do with worms. Ringworm is actually a fungus. To add to the deception, several conditions look like ringworm but are something else entirely.

Ringworm - Not a Worm

The medical term for ringworm, dermatophyte, means a fungus that infects the skin. Once an infection (dermatophytosis) is established, the offending fungus announces its presence, usually in the form of an itchy, circular pattern of hair loss (the “ring”), sometimes accompanied by redness or a rash. In horses, it commonly occurs on the girth and saddle areas and can spread to other parts of the body.

Related: Diseases with Horses-to-Human Transmission

The skin reactions are easy to see, but the multiple species of dermatophytes that can infect horses are less than forthcoming about their everyday presence, hiding out of sight in the soil, or stowing away on objects or materials such as shared grooming equipment and tack. The fungus can be present on horses for a few weeks before clinical signs appear and can spread from one horse to another by direct contact. These sneaky tactics mean that ringworm can also infect people or other animals, spreading through otherwise seemingly innocuous routes.

ringworm in horses, fungal infections horses, crusty sore patch horse

The typical circular ringworm lesion with scaling or crusting, and a bald patch in the middle. Disinfecting the barn and arena is important in the control of ringworm, which is highly contagious. Photos (above/below) courtesy of WCVM.

ringworm in horses, fungal infections horses, crusty sore patch horse

Diagnosing ringworm involves taking a hair sample for a fungal culture in the laboratory. It is important to confirm the diagnosis, as other conditions can mimic ringworm in appearance, and the most effective treatments can be very different.

Ringworm Look-Alikes

Although ringworm is a common equine skin condition, it is not the only possible cause of hair loss and rashes. Ringworm look-alikes include bacterial (staph) skin infection, occult sarcoid (a type of skin cancer), and alopecia areata, a rare autoimmune skin disease where the body’s immune system mistakenly attacks the hair bulb. Rain scald (dermatophilosis) may also resemble ringworm.

Ignore It and It Will Go Away?

In some cases, ringworm resolves on its own without treatment, but it can take a month or more. Therefore, it is important to treat horses as soon as possible to limit their discomfort and avoid spreading the fungus to other animals and people.

Related: Caring for Tack and Riding Equipment

Treatment may consist of antifungal shampoos, dips, or topical therapies. Oral treatments such as griseofulvin and terbinafine are also available, but potential unwanted side effects, such as liver damage (evidenced by elevated liver enzymes), and birth defects, should be taken into account. It is important to follow veterinary and product instructions to get the maximum effect. Treatment also extends beyond the horse. Tack, blankets, grooming equipment, and other potentially contaminated surfaces should be disinfected to limit the spread of disease. If a known soil-borne fungal species is identified through laboratory culture, the ground and/or stall floor should be treated with bleach to try to eliminate the organism.

Whereas bacteria usually grow quickly in culture, fungi grow in the laboratory over a period of weeks. It is not uncommon for clinical signs to resolve before the results of the culture are known. However, the results may still be beneficial for cases that are difficult to treat, or to provide information on appropriate treatments for other affected animals on the premises and limit the potential spread to humans.

Summer Sores (Habronema worms)

Unlike ringworm, summer sores are actually caused by worms (scientific term: nematode). Habronematidosis (formerly called habronemiasis) is a disease attributed to any of the three species of Habronema worms that parasitize the horse: H. muscae, H. microstoma, and H. megastoma. Adult worms live in the stomach where they typically cause little reaction. Females lay eggs and the larvae are passed in the feces where they are ingested by the maggots of either the housefly or the stable fly, which serve as intermediate hosts. The normal life cycle is complete when flies deposit the infective larvae around the horse’s lips, where they are subsequently swallowed and the larvae develop into adults (worms) in the stomach.

Summer Sores horses, habronema worms horses, mucous horse's lips

Mucous membranes, such as the lips, are the areas most commonly affected with summer sores. Photo courtesy of UC Davis

Summer sores develop when the larvae are deposited in previously damaged skin or mucous membranes such as the lips, nostrils, or genitalia where they cannot complete the life cycle and instead cause a local inflammatory reaction. The lesions themselves consist of areas of ulceration that usually contain small, gritty, yellow nodules (sulfur granules).

The disease is seasonal, first appearing in the spring and in most cases spontaneously regressing in the winter months. Effective treatment involves a veterinarian cleaning the wound (debriding), often along with administration of ivermectin, corticosteroids, and topical treatments. The wound should be covered, if possible, and it is important to follow stringent fly management practices. Once the disease develops in a horse, it will usually recur every summer. A genetic susceptibility has been proposed in some horses, especially those affected annually, but further research is needed. Adhering to strict fly preventative measures can minimize recurrence, but it is difficult to completely prevent recurrence of summer sores. 

Related:  Scratching the Surface of Equine Skin Disease

Related: 10 Things to Know About Equine Dermatology

Printed with the kind permission of the UC Davis Center for Equine Health. The UC Davis Center for Equine Health is dedicated to advancing the health, welfare, performance and veterinary care of horses through research, education and public service.

Main Photo: iStock/Middelveld